1. Field of the Invention
The invention relates to a process for preparation of cationically stabilized and water-redispersible polymer powder compositions, where one or more ethylenically unsaturated monomers are subjected to free-radical polymerization in an aqueous medium in the presence of a non-ionic protective colloid and/or of a non-ionic emulsifier and then dried.
2. Description of the Related Art
Aqueous polymer dispersions can be stabilized via dispersing agents which have properties making them active at interfaces. Water-soluble compounds in the form of protective colloids or emulsifiers exert this function. The stabilization can be achieved via steric interactions, for example via non-ionic polymers and emulsifiers, or via electrostatic interactions, the latter taking place via anionic or cationic functional groups in polymers or emulsifiers, or via an appropriate combination thereof.
When redispersible dispersion powders are prepared whose monomer base comprises vinyl esters, vinyl chloride, (meth)acrylate monomers, styrene, butadiene and/or ethylene, these being used in a wide variety of applications, for example as coating compositions or adhesive compositions for a very wide variety of substrates, polyvinyl alcohol is preferred as protective colloid during polymerization and drying, since polyvinyl alcohol gives the powder effective stability with respect to premature coalescence of the particles. However, if relatively large amounts of dispersion powder are used in the final application, the polyvinyl alcohol content can have an adverse effect. By way of example, there is an adverse effect on the workability of polymer-modified hydraulically setting mortar compositions based on cements or gypsum, the reason being high trowel adhesion and high viscosity. High polyvinyl alcohol content also brings about low flexibility of the adhesive bonds produced by the mortar composition.
Possible alternatives used instead of polyvinyl alcohol as drying aid are formaldehyde condensates of melaminesulphonic acid (DE-A 2049114), of naphthalenesulphonic acid (DE-A 2445813), and of phenolsulphonic acid (EP-A 407889), or aliphatic aldehyde condensates of dihydroxybiphenylsulphonic acid (WO A 2005080478). Lignosulphonates (EP-A 149098) or polyelectrolytes functionalized by anionic groups (EP-A 073807, EP-A 982351) are also used. A disadvantage of these alternatives is the large amount used of drying aid, this amount being needed in order to obtain a blocking-resistant and rapidly redispersible powder. Furthermore, the brown coloration that occurs in the mortar composition with sulphonic acid formaldehyde condensates or lignosulphonates is undesirable in certain applications.
Cationically stabilized dispersion powders have been previously described in the prior art: WO-A 00/05275 discloses a process in which cationic monomers are polymerized in an aqueous medium and other monomers are polymerized in the presence of the resultant polymer which has been formed in situ. The polymer dispersion thus obtained is then spray-dried in the presence of polyvinyl alcohol. WO-A 00/05283 describes a process analogous thereto, and here the control of the process is such as to form copolymer particles with heterogeneous morphology. However, a disadvantage of the process is that large amounts of polyvinyl alcohol as protective spray-drying colloid are needed for preparation of the dispersion powder by means of spray drying of the polymer dispersion initially obtained, the result therefore being occurrence of the abovementioned shortcomings when large amounts of powder are used.
EP-A 0770640 describes a process for preparation of redispersible polymer powders which dries an aqueous polymer dispersion whose dispersed polymer particles have a negative or positive electrical surface charge, where the drying aid selected comprises a polyion whose surface charge opposes that of the dispersed polymer particles. A disadvantage of this process is that partial coagulation of the polymer particles can occur during mixing of dispersion and protective colloid prior to the spray-drying process, and the powder obtained can therefore have relatively poor redispersibility.
WO-A 98/13411 describes the preparation of redispersible dispersion powders which are obtained via spray drying of an emulsion polymer which contains anionic groups, preferably COOH groups, with amphoteric polymers, preferably casein or sodium caseinate. The amount of the protective colloid used in the spray-drying process is stated as from 2 to 10% by weight, but completely tack-free, flowable powders are not obtained until 10% by weight is reached. A disadvantage of casein as protective colloid is its natural origin and the variable product quality associated therewith, which can make the quality of the powder inconsistent. Furthermore, casein, like all proteins, eliminates ammonia under alkaline conditions, causing VOC pollution (VOC=volatile organic compounds).